An antisense drug developed by Prosensa and GlaxoSmithKline (GSK) has improved walking ability for patients with Duchenne muscular dystrophy (DMS) in a phase IIb trial.

The positive data on the candidate, called drisapersen, ups the ante in a race between Prosensa/GSK and rival Sarepta Therapeutics to bring the first drug treatment for DMD to market.

The top-line trial results were revealed at the RNA & Oligonucleotide Therapeutics Conference in Cold Spring Harbor, US, yesterday, and revealed that drisapersen achieved a clinically meaningful improvement in the six-minute walking test in DMD patients compared to placebo after 24 weeks of treatment.

By the end of the study period, patients on continuous drisapersen were able to walk 35 metres further than those on placebo, with the difference maintained up to 48 weeks, although intermittent treatment with the drug conferred no significant benefit. There was also no difference in muscle strength compared to placebo for either drisapersen regimen.

Neither GSK nor privately-held Netherlands firm Prosensa put out a statement on the disclosure of the data at the time of writing, and analysts remarked the timing is a little odd given that they had previously said no results on the drug would be available until a phase III trial was completed later this year.

It has been suggested that the companies have brought forward their presentation in response to the news that Sarepta hopes to file for accelerated approval of its eteplirsen candidate in the US based on the results of its own phase IIb study, which was completed last July with positive 72-week data disclosed last week.

One immediate impact of the GSK/Prosensa announcement was a lift to Sarepta's shares, as eteplirsen has a very similar mechanism of action to drisapersen and the latest data validate its own programme.

Both drugs work via a process known as exon skipping, which partially corrects a defect at exon 51 in the gene coding for a protein called dystrophin that is the underlying cause of symptoms in DMD and other forms of muscular dystrophy. The result is the expression of a shortened form of dystrophin that restores some of the function of the healthy protein.

Analysts have suggested both drugs could achieve sales of up to $500m at peak, with some suggesting eteplirsen may have an edge in the marketplace in terms of safety, although clearly the clinical datasets for both agents remain small at the moment.